| The purple soil area in central Sichuan hilly area located on the upper reaches of the Yantze river. This area affecting the soil and water loss and Ecological Environment of Yangtze River watershed and Three Gorges area directly has become the second largest erosion area after the Loess Plateau. In this study, Chen Jiawan small watershed was selected as the study area. The Applicability and accuracy of Crop growth module of Soil erosion process model WEPP to the simulating crop growth was studied based on field survey data. Test of soil water infiltration ability in study area was carried out in order to make reference to erosion and Run-off simulation of WEPP. The establishment of the model database was carried on this basis, and simulation of soil erosion and Run-off was carried in the study area using the WEPP model, the applicability of the WEPP model in the purple soil area was analyzed, and soil erosion intensity was defined to become a useful explore and reference of development of soil and water conservation measurement and assignment of landuse. The main conclusions of this study are as follows:(1) The crop growth module of WEPP simulate the crop height and cover with accurate, the simulation of the growth trend of the crop was in good agreement with the actual trends, and the model over-estimated the vegetation cover but described the trends of change of the vegetation cover properly. The model simulation of shrub leaf area with the actual value is closer and the change trends is consistency. Only in the leaf area index simulation there are obviously over-estimated phenomenon exists, but it can also predict the change of leaf area trend properly.(2) The infiltration rate of the Purple Soil showed a decreased trend with time increased. In the 0-10min period, the decline speed was most fast; after 25-30min, the infiltration comes into stable period, and the horton equation can be used to describe the process of infiltration properly. The soil bulk density and soil infiltration capacity was negatively correlated, with soil bulk density increasing as the soil infiltration capacity decreased. In forest land within the different slope, soil infiltration rate variation showed as upper slope> middle slope> down slope; and in different slope aspect, the steady infiltration rate performance as semi-shady> semi-sunny> sunny.(3) The WEPP model can simulate the Run-off of the small watershed accurately, but with under-estimated phenomenon The relative error is -0.132, the Nash-Sutcliffe index is 0.723, the coefficient of determination is 0.898. However, the model simulation will be biased in heavy rainfall process. The model simulation of soil loss under single storm was not good and there are over-estimated phenomenon. When some soil conservation plan was made as a simulation scene, the simulated soil loss has been changed obviously. When channels was increased and described precisely but with little change. When the forest land was changed into crop land, Soil erosion in the study watershed is slight erosion according to the "Standards for classification and gradation of soil erosion", but still obviously higher than soil loss tolerance for purplish soils.
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